You're going to want to open the gfx23 file, and you'll have to count the different items. They have 2 icons per set, so for every set of one item, that's one. After you get to the end, follow the numbering pattern and add your icons, then add one to the last one you counted and use that as the
icon ID. So say 73 was the last one you counted, you add your icons and then use 74 as the ID for your item's icon.
As for the non-standard error, what file extension did you use for your images? If I remember right, .bmp is the only one that will actually work.
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May he now rest under aegis of mirage -
As the sands slowly turn to Elysian fields