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