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323-Freak:
This is a very old grievance and certainly something that should be fixed at the module level.
I don't have a code solution, but I do have an idea you might try if you have some experience playing with PHP. Look for some of Catzwolf's modules and in the file include/functions.php look for a function called xoops_createthumb.
You might try adding the function to the forum code, so that when an image is uploaded in a comment, it gets resized once it reaches some width defined in the function call.
Of course this should also be included in the to-do list of the module's developers.
Cheers.