It's not that they're a bad thing to have, and on the ZDoom forum it is quite a courtesy to put your image inside spoilers so that someone doesn't have to scroll down several pages just to see relevant content in a post. However, here, that little issue is automatically taken care of. Images are automatically shrunk and they are clickable and a user can click on your thumbnail for a bigger image. (Javascript is required for this functionality, though - just as it would be required to preview your image in the first place anyhow)
Since [spoiler] is a custom tag, however, that functionality breaks, and we are left with only a thumbnail that does nothing when we click on it.
It is not standard for me to edit your posts to repair this functionality, but when it happens I'll try to leave a note letting you know I did.
So please, by all means, post an image as big as you want and don't worry about spoiler tags. This forum will shrink them down for you.
If it's a habit, don't worry about it, I'll correct it if it's an issue for your particular image.(P.S. if someone does spoiler their image and you want to see a larger size, simply right click on their image and click "View Image" - the image will automatically open as its own page this way - you can simply use your back button to return to the post)