Creating thumbnail images requires three things: a PHP graphics extension; sufficient memory on your server; and sufficient CPU power and/or time to allow the process to complete. You can check these using information shown on your kiwitrees page Administration > Site administration > Server information.
PHP graphics extension
Kiwitrees uses the “GD” graphics extension in PHP. Your server information should therefore list GD, and show it as GD support = enabled.
For gd, your server information should include a section for GD, with an entry of “GD support = enabled”.
To process an image from full size to thumbnail, GD requires memory of 4 bytes-per-pixel. An image that is 5000×4000 pixels will require 5000×4000×4 = 80MB (GD). In addition, the rest of the script requires approximately 20MB. So generating a thumbnail of a 5000×4000 pixel image will require at least 100MB. Your server’s “memory_limit” must be set to a figure higher than that.
CPU power and/or time
Image manipulation requires a lot of server resources. If you have very large images, and low memory limits, then you will need to arrange, perhaps with your web host, to increase any limits on these..