Fix T103516: max image size for previews is set too low

Manager had a limit of 10 MB, but the Worker can produce images that are
larger than that (even after down-scaling the image). I've bumped the
limit to 25 MB, which should be enough (it's 2x the bug reporter's file
size).

internal/manager/last_rendered/last_rendered.go

@@ -15,7 +15,7 @@ import (
 
 const (
 	// MaxImageSizeBytes is the maximum size in bytes allowed for to-be-processed images.
-	MaxImageSizeBytes int64 = 10 * 1024 * 1024
+	MaxImageSizeBytes int64 = 25 * 1024 * 1024
 
 	// queueSize determines how many images can be queued in memory before rejecting
 	// new requests to process.