Picture formatting in Sigil
 

I am testing on three devices.
apple computer - iBook
Nook-Simple Touch
Kobo-Touch

by just inserting a picture file into Sigil and saving the epub, both the apple and the nook auto centered the picture upon display, but Kobo did not.

I went and centered the pic in Sigil and that fixed the problem...but
On Kobo, a large "1" is being displayed under the picture, and I need to get rid of it.

Any ideas why that is? Neither the Nook, nor iWork display this random number under the picture.

Lastly, when I save my jpg, it give me a choice of least to best as far as quality goes. The least is only 10% the size of the best. Upon inserting each of these images into sigil and saving the epub, the epub files are also 10X different, but I see no visible difference when I check on the three devices. Am I missing something here? Should I just use the smallest image. They all display beautifully and one would be much much smaller and therefore desirable.

Presumably, the pictures with the larger resolution are so good you cannot even see the detail on an ereader. ;)

Yep, your average ebook does not need such fine quality pictures, not unless you happen to like bigger filesizes for some obscure reason.

It is not the resolution....I can adjust the resolution 100, 200, 300, etc.
It isn't that. When I save, I have a slide bar from least to best, and there is a 10x difference in size of file, but when I open the file, it still shows the same resolution.

JPEG is a 'lossy' format.

When you are saving the image from some kind of editor, you can usually select a degree of compression, or quality ("least -- normal -- best").

The lower the quality, the more highly compressed the image will be, but smaller file size

Not my field, but I'd expect the reader's dpi, as well and the horizontal/vertical pixel count to get involved also

Edit -- this is an interesting read about covers, but I think it would apply to any picture

http://www.thebookdesigner.com/2012/...of-resolution/