I definitely had my own problems with my images when first working with Bootstrap. The big thing that helps is going through the documentation. Bootstrap’s is really helpful. For example, Bootstrap Components - Thumbnails. I would also strongly suggest taking a detailed read over the containers and grid system it uses (see CSS at the top).
You can’t nest containers and container-fluids without getting really wonky results. But (the awesome thing) you can nest your rows and columns and even offset your columns, which you need to utilize to make your thumbnails look right rather than specifying a width and height (when mobile users won’t have as many pixels to work with). Either that, or you have to set different height/width for smaller screens, which is a bit more complex.
You also don’t need to put the col size for each size screen unless you’re giving different numbers to them (like having something take up 3 cols on med screens, 6 cols on small/tablets and 12 cols on xs/phones). Giving those variances between devices when appropriate can really help your content look right on all devices.
Once you get that grid system down, layouts become much easier. You can kind of draw them out at first if it helps.