Deki Wiki uses ImageMagick to create thumbnails from uploaded images. To be thumbnailed, an attachment must be smaller than the size defined by
files/imagemagick-max-size
By default this is: "268435456" (2.5MB)
The attachment must also have a file extension listed in
files/imagemagick-extensions
By default this is: "bmp, jpg, jpeg, png, gif, tiff"
You made edit these values by going to Control Panel -> Configuration.
Note: The configuration screen is introduced in Hayes+ 1.8.2. For previous
versions, you'll need to make a direct database update into the Config
table and restart dekihost.