Image size and file size
Firstly, there is a difference between image size and file size. Image Size refers to the dimensions of the image measured in pixels. File size refers to how much space the image takes up on
your hard drive in kilobytes, megabytes, gigabytes etc.
Resolution — ppi or dpi means the number or pixels or dots there are within an inch. The higher your dpi or ppi the higher your resolution and the finer the detail of your image. But at the same time the higher your file size. This becomes important for electronic images because sometimes there are limits imposed on the file size you are allowed. For instance, on Amazon, the maximum file size for a whole eBook is 50MB.
Always start off with high-resolution images if you can.
When you are sourcing images to insert into your books it is important to begin with high resolution images even though this increases the file size because you can make a copy at a reduced size to accommodate those times when your file size needs to be lowered but you can't scale up a small image and expand it without
So if you start off with a low-resolution image thinking it will be adequate for your eBook and then decide to publish a paperback of your book afterwards the low-resolution image will pixelate when you try to enlarge it to use in your paperback.
For example, in the image below you will see two versions of the same 6 x 9-inch image for my book in Photoshop. I am viewing the one on the left at full size and the one on the right is zoomed out to 25% so that they look the same on the screen. They are both 6 x 9 inches but the one on the left is for an eBook and has a resolution of 72 pixels per inch and the one on the right is for print and has 300 pixels per inch.
The file size of the one on the left is only 820 Kilobytes but the one on the right is 13,9 Megabytes. If you scrutinize the image you will notice that both covers will look fine on the screen where the screen resolution is only 72 ppi anyway. Except that the righthand image is overkill for a screen because it’s file size is huge.
High-resolution images for print and lower resolution for eBooks.
You might get away with using an image with a large file size if you only have the one image in your eBook, like your internal book cover, but if you have multiple images you will have to reduce their size because Amazon charges a per-gigabyte fee for delivering and also has a 50 Megabyte limit per eBook.
Conversely, the eBook image on the left, at 72 ppi is roughly 25% of the size of the paperback image on the right so if I were to try to enlarge it to fit the same space, its 72 pixels per inch would have to grow to fit the space, eventually becoming visible to the naked eye.
So my best advice is to start with images that are at least 300 ppi in case you want to use them for print later but when putting them on the web or using them for your internal ebook cover, reduce them to around 72 ppi to bring down the file size which will save bandwidth, and load them faster. Learn to resize images in this free course.
Always ask your image designer for any original images you have bought during the design process, especially when dealing with book covers, just in case you want to make changes and your designer has moved on or you have fallen out. (Ask for the Photoshop files as well.)
Then, save your original images and store them in a safe place. Remember, digital images that have a very high resolution take up a lot of storage space on your computer so you might like to store them in the cloud or on an external hard drive.
What file type is best for your project
When you save your regular images the most convenient file type is a .jpeg (.jpg) because it is compatible with all the servers and due to its small size will upload faster. The disadvantage is that since .jpeg is a lossy format you may see compression artefacts, especially in the regions around your text.
To show you what happens, I've cut the image below in two and saved the half on the left as a .jpeg and the half on the right as a .tiff. Then put them together again. Look closely around the arrow area and you will see what compression artefacts look like on the .jpeg half. The .tiff section on the right-hand side is nice and crisp. (The quality of a .png will be similar to the .tiff)
.Tiff and .png files are lossless and retain all of their quality but their file size can be up to twice the size of a maximum quality .jpeg of the same image. So it depends on where your image is going to display and how text-heavy it is. You have to test this for yourself because each image is different.
Here is exactly the same split image zoomed out and you can barely see any difference, so for this task, I would use a .jpeg.
A good rule of thumb is to go for .jpeg if your image is mostly made up of photo type images and resort to .tiff or .png if your images are text heavy and you are seeing compression artefacts. Also if you ever add images with lots of lines or text like tables.