Just back from my 3 week trip to Italy.
Made 5000-odd photos.
Using my posted scripts they are easily sorted.

But some are crooked, lets fix them.

Not a straight horizon:

Lets open this photo in Gimp

Easy to fix:

• Open in Gimp
• Go to measure tool
• Orientation – auto and transform layer
• Select the horizon on the left (do not release the mouse button) and draw to the right
• Press straighten

Easy ..

Another fix below : Perspective

Sometimes you want to look straight at a picture, but when taking a picture there is something preventing this.

• Other tourists won’t move away from a information sign, so you have to take the picture from an angle.
• The image is far up a wall.
• Or as in example below, the Roman mosaic is on the floor, no way to take a picture from above.

• Open in gimp
• Select from tools > Transform Tools
• Handle transform or press SHIFT-L
• Select a corner and drag, repeat until satisfied