So guys, i've made this guide for importing images as sprites into sonic games so let's start. First off you need to search the sprite that you want to import. for example: We want to import one of these signpost in the game so first off we need to cut the image we want so open it in paint, click on the border on the image and crop it. Then we need to remove the background, otherwise it will became sh*t, so use a computer program or a online app, i will use http://remove.bg for removing the background, because is a flexible online app. After removing the background save it and open it in paint. Without closing Paint, open SonMapED, load art and mappings of the signpost, select a sprite then click on view>info and check for height and weight. Go back to paint and resize it as specified in the height and weight values. Save it the go again to SonMapED and go to file>import sprite over active frame. Then save it as a copy. Open Sonic1.asm and search for artnem/signpost.bin and replace it with artnem/(your new file name here).bin. Click on build.bat and then you're done. Thanks for reading.
First, if you plan on editing any art, I'm just going to say that Gimp, paint.net, or even any dedicated pixel art program are incredibly valuable tools and totally worth learning for the purpose of saving time in the future. As a simple and easy to use paint is, it lacks necessary features that require workarounds like the one you suggested. Second, "import sprite over active frame" is an archaic time sink that no one should use given the other options available. Flex 2 auto creates the mappings for you when importing a sprite which is way easier. I recommend just using Flex 2 instead of SonMapED. Finally, you didn't specify compressions, so someone following your guide is likely not going to know that artnem contains files which are nemesis compressed, and when saving an art file, compression needs to be specified.