I'm in the middle of making a set and I have a few question in gimp how do i mirror arms and legs and how do I get rid off the line that appears in the back of your head texture?

If there is a line on the spinny head preview, just ignore it. It will not show up ingame.

Arms and legs you have to mirror a specific way, matching the templates found in the Textures section of the ToriShop.

The line in the back is because of color on the edges of your drawing because its a flat image but it gets wrapped around so think about it, if theres a line at the edge or no color thenn there will be a line on ur head, also to mirror press edit>transform>flip horizontol, and itl mirror the image

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The part about mirroring is completely incorrect.
To mirror Biceps and triceps, Select the whole image, and mirror it. Then cut off 24 pixels from the left side and move it to the right, then move the rest of the image to the left.
To mirror legs and thighs, select the top half, and rotate it 180 degrees and mirror, and then select the bottom half and do the same thing.
To mirror hands select Top Bottom Front Back and mirror, then rotate the in and out 180 degrees.

That should be it. And btw, Ezeth made a nice tutorial on this, use the search button