One of the great features of Concrete5 is its built-in File Manager. I would strongly recommend utilizing the File Manager for your edit interface. Yes, it does add an extra step when users need to upload new files, but it's usually worth it because often the user already has the file uploaded so you want to allow them to choose the file from the file manager instead of re-uploading it. Also this gives them flexibility to change the image in the future from the file manager (like upload a replacement if they want to change it, or change the title or other properties), whereas if you only allowed uploads directly from your block interface, they don't have this flexibility to change it as easily in the future. Also also, there is an advantage to having a consistent interface for file uploads -- once users know how to use the File Manager, then they can just assume it always works like that and not have to keep learning new things.

All that being said, there are three general styles of edit interface I've seen in the C5 world:

1) Choose 1 image -- see the built-in "Image" block for how to do this. Very straightforward and easy to do, but only works for 1 image.

2) Choose a File Set -- this is the easiest approach for you to develop, because you just need a dropdown menu showing all file sets, then when you display the gallery you query all images in that file set. This is not always ideal for the user though because it presents another layer of abstraction (and we're already adding one with the File Manager, so adding another on top of that might get confusing). I have a free addon in the marketplace called "Simple Image Gallery" that uses this approach. You should download that and check out the code to see how I did it. Note that I have one complicated thing in there which is I added a menu item to "go to file manager", so if users get to the edit interface and don't have a file set already made for this gallery, they can go directly to the file manager to create one without having to close the block edit dialog. If you want something even simpler which isn't as user-friendly, but the code is easier to understand, I'm attaching a bare-bones addon I've made which only shows a dropdown menu for the file set and doesn't have that "go to file manager" functionality to confuse matters. Note that this sample image gallery uses the "Coin Slider" on the front-end, because it was the simplest interface I could find for a jquery slideshow plugin, so the code is easiest to understand. Shouldn't be too hard to figure out how you would swap that out with another slide show on the frontend though.

3) Choose multiple individual images. See the built-in "slideshow" block for an example of this interface. This is definitely the easiest for end-users (especially because they can actually choose multiple images in the file manager by checking multiple boxes and picking "Choose" from the "**" dropdown menu at the top of the list). Kind of the best of both worlds combination of individual files and file sets. BUT it is much more difficult for you as the developer to code up. You're going to have to dig through the slideshow block code to see what's going on, and trust me it's not going to be easy :)

Good luck!