[PatNY]

Claire, the two big things that will impact file sizes for jpegs are --

1) resolution -- ie, the physical dimensions of the image. For example, if you have an image with a resolution of 1200 x 800 pixels, reducing it to 600 x 400 pixels will cut the size considerably, assuming the compression level used to save the images are roughly the same.

2) jpeg compression level used -- this is an adjustable setting in the program you are using to output your jpegs. Raise the compression level to reduce file size. The actual adjustment is often called "Quality" in which case you lower the "Quality" to reduce the file size.

You can do either 1 or 2 or BOTH to reduce the file sizes of your jpegs. For image heavy books, I prefer not to touch the resolution at all (as images will then appear smaller on your device) and instead simply adjust the image compression.

To do that, I "explode" the epub using Calibre's tweak epub feature. Then I select all the jpegs in the epub's image folder and drag and drop them onto the interface of Fotosizer (a free program). I then use a compression setting of anywhere from 40 to 60 in Fotosizer to resample the images and save right back to the original folder. I can usually get up to 75% space savings (sometimes more) in some image books.

Two things to note: Compression levels (or quality settings) are different with every program -- ie, 60 in Fotosizer is different from 60 in Photoshop which is different from 60 in Irfan. Just keep that in mind if you are trying out different programs. Test the output to make sure the file size is what you want.

Another thing -- Make sure you backup your original epub before resampling images, AND only resample images once. JPEG is a lossy format, which means every time you re-save the jpeg you are losing some quality. So if you have to redo something, go back to the original images.

DPI will not impact file size ... that just sets the size of the output document if printed.

--Pat