Basically; Lazarus is insisting that it can't find a certain file in the "uses" section, even though I've triple-checked that it's in the search path. The exact same setup compiles without any problems on Windows (this does mean that if I can get cross-compile to work, this could be an alternative option, but as I mentioned I've had no luck with that either). I might take another look and see if I can get it going at some point, but I'm kind of making it a low priority for now.
EDIT: At a quick glance, it seems the problem here *might* be caused by my unfamiliarity with Linux; in particular, not taking into account its case-sensitive nature with filenames. I'll check and see if this is really the issue...
EDIT: Indeed, adjusting for this got the code to compile! This does still leave a major issue in that the compiled code doesn't seem to actually work (it displays a blank window, and doesn't resize / go full screen as it should), but if I've at least got it to compile, then that's a start.
EDIT: Okay, I've got the resizing of the image display handled; the problem comes from that when an image is displayed on the window, the OnKeyDown event of the form applies under Windows, but of the image applies under Linux. Goes without saying that I have no idea why this happens; but simply making sure *both* handle the event properly solves this issue. For some reason though, it doesn't actually resize the window itself properly... and it still won't actually display the image under Linux. Through some other test code I inserted, I can confirm the problem comes from actually *loading* the image; not from displaying it.
EDIT: Tried modifying the code so that it uses a standard Lazarus class to load the image, before transferring it into the addon graphics package one. The result was the app (which I should mention - at this point, all it does is display a static image and resize it according to zoom / full screen) taking FOREVER to load. Once it loaded, the image display worked fine (still need to address the resize issue). I believe this is due to the standard class being known to perform very slow on Linux; but another weird thing happened here - the result was inverted colors when tested on Windows, yet correct color displays when testing on Linux.
All in all, it looks like indeed, cross-platform is going to be far too much of a hassle if I intend to use anything Delphi-like to do it. With this in mind, I go back to what I said before - NeoLemmix V2.00n is still going to be Windows-only. Given this, I'm considering just working with Delphi, as there's quite a few things that annoy me about Lazarus's interface... I would assume that should someone be willing to get around these issues, converting it to Lazarus would be the least difficult part, since Lazarus 99% automates that.