Is there an image library limit? Like how many pictures or how much storage?
As far as I'm aware, there is no hard limit on the number of images you can upload into LibGuides using the Image Manager, so you should be able to upload whatever image you'd like. As a general guideline, each site gets 2GB of image storage space.
Generally speaking, an image file you'd add to a web page isn't that large, so you should have plenty of room to add whatever images you'd like. For example, in the LibGuides FAQ I have around 200 images...in the Springshare Help site I have around 250. And that's just my account - that doesn't include whatever my colleagues have uploaded. Looking at the images folder I have on my computer for all of my Springshare related images (screenshots, icons, whatnot), there are 1,413 files for a total of 38.3MB...and I'm sure I'm not done. ;)
When thinking about individual image file size, you want to keep it as small as possible so that it loads quickly on your page. The larger the file, the longer it'll take. So uploading a 5MB image file means it's going to take a long time to load on the page. That file size is great when you want to print images...but isn't so good for the web. As far as web standards go, the general recommendation is that an image used on the web be no larger than 100kb. For more info on this, check out our FAQ: What image size should I use?