It bugs me to no end to see artists stretch their work to fit into provided frames (perhaps that of memes, thumbnail previews, etc). Even more so if it is clear that the artist is actually very good and one would suspect they would know how horribly hideously ugly this is.

I cannot say it enough:

Cropping

Cropping an image will mean you have to sacrifice some detail around the edges of the image, but this is usually managable - and far preferable to stretching.All digital art programmes, including the old MS Paint (yes!) will enable you to crop an image. You don't want it looking like Dreadno here. (News appears not to support image macros, so clik the thumb for a big version.)

You may feel like stretching is the better option because you get all of the image in the frame, but SERIOUSLY. What is the use of having the entire image there if you can't SEE any of it? I've used only a very mild example of Crop/Stretch here, but surely even you can see it is worth sacrificing some of the image to make sure at least MOST of it is visible and discernible.

Don't ruin your image with stretching.

If you can't crop, matte.

Sometimes an image shouldn't be cropped because the most important details are at the long ends. Perhaps text, or close-up previews you feel are essential to the image.

This is not an excuse to stretch.

If you find yourself in this situation, then pick a colour appropriate to the background of your image as a 'matte'. Resize your image without stretching until the longest length fits into the designated frame. Then fill up the empty space around the image with your matte colour:

Despite the empty space this will create, having to deal with a little extra space is a small sacrifice to make sure people can still actually see your image.

SAY IT WITH ME NOW:

If you cannot CROP, then MATTE.
NEVER EVER EVER EVER EVER STRETCH
OR I WILL SEND DUCKS
TO YOUR HOUSE
AND THEY WILL STAB YOU
IN THE FACE