Texture seams. Look in guides categories to how to Vam to Blender. In Blender if you (have a open help pose it help) there is a texture paint mode on the top, then you select stamp tool to fix seams.
Forgot to say, you should save in tiff has a working file between Gimp and blender and then only export final in jpg. Blender is terrible for saving in jpg, even when quality is 100%.