I went in-world to see if I could find examples of what I'm talking about. Found one. Dunno if this is the problem others are talking about.
What I did was copy the image so the top and bottom edges are close and circle the area to look at. Notice the black line on the white edge.
Since you can't change repeats you can't fix it. KittyCatS would need to adjust the repeat (vertical in my image) to be a bit less than 1 so the image file extends a bit beyond the edges of the prim face. They'll want to leave the other repeat (horizontal for my image) exactly 1 so your textures seamlessly wrap around the neck.
All you can do is make sure the pixels at the top edge exactly match the pixels on the bottom edge. I see a number of people are doing just that. The post just above says they did that, in different terms; making a solid-color border, the same color on the top and bottom of the image.