Is forcing full-screen a bad practice, or is this acceptable?

I think “forcing” is the keyword here. If the user doesn’t know clicking an image will fullscreen it, I would say thats problematic.

One solution could just be adding a warning that images will be opened in fullscreen.

A better solution might be to show a fullscreen icon when hovering over an image as it would describe to the user what might happen if clicked.

No. Never force UI layout on users when forcing it will potentially impact the user's ability to use their system as they would if your application (web or otherwise) was not there. Doing this type of thing is a very reliable way to seriously irritate your users.

In this particular case, I wouldn't even suggest fullscreen though. Most users don't mess around with it for anything in a web browser because it has a negative impact on their ability to multitask effectively even if they're using multiple virtual desktops/workspaces, and you end up gating a huge amount of browser functionality behind keyboard shortcuts.

As far as handling the UX benefits more sanely:

• If it's just screen real-estate, stop owrrying so much about it. There's no reason that you need to be full screen in any well designed website or app that isn't an actively immersive game or VR-type thing.
• If it's just because 'it looks better', then stop thinking like that. You're designing a website, not an OS, not a desktop environment, not a web browser. It's not your job to worry about how your site is framed on a user system.
• If it's due to actual UI differences between the normal and full-screen versions, get rid of those differences (unless they're just based on viewport size). Changing UI between full-screen and windowed modes is almost always a bad idea for an app.
• You mentioned it's for a photography site, so I assume the primary issue here is tro try to open images at the correct aspect ratio and as close to the correct resolution as possible. If so, do like most video sites do, start at less than full-screen, and for each image have a button you can click to open it full-screen, with a clear method to exit full-screen beyond the normal F11 option provided by the OS/desktop environment. The escape key is a good option for this as it's reasonably standard on websites for this type of thing.

Why not a well-worded suggestion? If something just went full-screen on me I would be upset.

Hell no, don't you dare!

Sounds like a bad idea. Is it just for the screen real estate? Because most people's browser windows are already near full-screen size.