Pictures don't compress very well -- because they are already compressed, of course -- so every kilobyte added is another kilobyte to the final ebook.
As for size limits on attachments here, unless it grows beyond 20 MB you're safe.
Use your best judgment.
Personally, I see no reason for high-quality images in an ebook, as E-Ink devices cannot render them in all their glory anyway. Most of my books only have a cover
and I am used to ebook sizes of ~500 KB.
If you want to save on image size, there are things you can do to optimize it. Various threads have discussed it (mainly in the workshop). The usual recommendations are e.g. for B&W images, to convert to gray colorspace, reduce the colors to 16 shades, strip them of profiles and comments, etc.
To be extra careful, you can run them through a time-consuming round of zopflipng.