the difference between PNG and JPG in drawings.
As you can see from your drawing I have 'filled' it with the pink colour - the four .png drawings are perfect in their outlines. The Me-410 has got all the extra pixels and shades of grey on the outlines that shows any pixel artist in this forum that the drawing was saved in the 'jpg' format at some stage. It is one of the first things you learn, mainly from experience. But in every drawing you create you should give it the 'pink' test to pick up all of the stray pixels that do come on to your drawing. To reverse the 'pinking' just select the white from your colour palette and click the fill button on any of the pink area.
You need to download a new Me-410 in its png format and start again with that drawing, keeping it in png format through all the saves. Then copy / paste it to your picture.
Learning something new is always frustrating, the 'old' hands make you feel stupid. We all go through it. I look at the drawings I did 7-8 years ago and I literally shudder at how awful they are. Now it is second nature to do the right things at the right time and the results end up pretty impressive.
Unfortunately there is no shortcuts to becoming a good artist, it is mostly time and hoping that others will treat you with some respect for trying. We have hundreds of members but less than 50 who contribute drawings to the forum regularly. Something the critics should remember!