Our gallery has a maximum file size of 300 kB, so you need to adjust your saving settings to meet that criterion.
Step 1. Save a full-size .pspimage format, to keep safe.
Step 2. Merge your layered image into a flat image (it will be temporary)
Step 3. Resize to 600×600 pixels using Image > Resize
Step 4. File > Save as… (1) Make sure you save in .jpg format (2) add a suffix like -600 to identify that image that has been resized and (3) adjust the compression value, if the File Size shows higher than 300 kB
Now, you are ready to upload to the gallery.