The texture imagefilename is not a power of 2, please resize it.
Image files referenced from a texture.txt file must be sized so that each dimension (height and width) is a power of 2. That is, 2, 4, 8, 16, 32, 64, 128 etc. Image measurements are always in pixels.
Note that this does not mean that the image must be square - the requirement applies to each dimension separately.
For instance, images could be 16x16, 16x32, 128x512 etc. Most any image editing utility will have the option to resize an image. Also, many image viewers can save an image at a nominated size.