So I know how to upload images to a thread, but how do shrink their size? For example in my build thread I do not want a massive picture of this bolt, because it is not that interesting.

 

resizing images is done with a photo editing program. i've used gimp but that was very long ago, they may still exist. windows and mac come with such programs already installed.

 

I literally just use MS Paint and resize them down to around 20% not the most elegant option but it does the job

Gimp does still exist though they stopped updating it 10 or so years ago. It's still one of the best free options that I am aware of.