A few things to keep in mind when working with textures:
- keep textures reasonably small, ideally below 1MB (files larger than 4MB will not work)
- make sure to use seamless textures for the best results
- textures should be square and sizes of 256, 512, 1024 or 2048px
Find more details on texturing in this article:
Also, posting a minimal version of your definition would help us a lot to understand the issue.