Manual:$wgMaxImageArea/en

Details
This value specifies the maximum number of pixels a source image can have if it is to be scaled down by a scaler that requires the full source image to be decompressed and stored in decompressed form, before the thumbnail is generated. This provides a limit on memory usage for the decompression side of the image scaler.

If set to, MediaWiki will not check the size of the image before attempting to scale it. Extensions may still override this setting by using the hook.

The limit is used when scaling images with any of the built-in image scalers, such as ImageMagick or GD. It is ignored for JPEGs with ImageMagick, and also when using the, extension as these situations do not cause excess memory usage.

Default is 50 MB if decompressed to RGBA form, which corresponds to 12.5 million pixels or 3500×3500.

File with dimensions greater than …
If you are uploading an image and you get this exception or a similar one:

Error creating thumbnail: File with dimensions greater than 12.5MP

You can increase the current value of the variable  in your settings.

Unable to extend cache
If you are uploading an image and you already increased the above limit but you still get this error:

Error creating thumbnail: Unable to extend cache

Then try increasing the following related limits as well: