I was messing round and found that LGM uses both, image alpha AND transparency color. Using both of them in the same time seems redundant and messy. I was going to suggest going the way GM8 did. Remove transparent attribute and everything that is connected to that. So have only alpha support and if someone wants to remove background color, then he just deletes that before importing (or in GM8 case, in the built-in editor). If <GM8 file is imported, then it checks if the transparency color is used, and if so, then just replaces the transparency color with 0 alpha. Its how GM8 does it.
Also, Enigma seem to save images in the exe using RGBA bitmap. So it takes a lot of space, shouldn't some compressed lossless format be used instead? Like .png? And what about Enigmas image file support? sprite_add seem to load only .bmp files. GM8 can load tens of formats as far as I know, and even if some of them are useless, I would love to use some. Like the stated .png. In every project I use external resources and for compression sake they are usually png and in some cases .jpg (where some quality loss is acceptable).
You could use some free C++ image library. Quick search revealed me http://cimg.sourceforge.net/
. Although reading about I found it only supports 8 formats natively, and some of them are ASCII files. So I guess it can't be useful. It can be extended with (png's, jpges and lots of others) imagemagik. Some other project might be more useful thou.
Also, I know that LGM sprite functionality is far from finished (or is it?), but I must ask for some additions/changes to be made more prompt than others. For example, you can only select one image in the load dialog. This is something similar to GM, but it also has Add Image and Add from strip buttons, which allows selecting other files too. Because now I can't add an animation which consists of several .png images. I can only add one. The second problem is that I can't paste into new frames. So in GM I usually just pasted image from a graphics program every time I changed it. Like selecting one frame, and then pasting the image and it added the image after the selected frame. Then I deleted the old one and that was it. In LGM you can "Add new frame", but that just adds a white frame to the sprite. You can't modify it in any way (as far as I know).