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[Scatty]

I'd also recommend this very handy picture converter program for any custom images (using it myself for many years):
https://www.irfanview.com/
Some options are slightly more advanced, but nowhere as complicated as Paintshop PRO and similar expert programs. There's not much of picture editing, but you can save in a big amount of different picture formats, and do some basic modifications to images.

[Smiling Spectre]

And if we are here, I, for myself, truly recommend XNView. Similar to Irfan, but as for me, more diverse (in terms of formats) with about the same capabilities overall. They bought me initially with their Spectrum pictures support and ability to browse raw data in graphic form (last one was removed in later versions, I believe). For some reason, Irfan never "clicked" with me. Personal bias, I believe.

[Japo]

+1 XnView.

used Irfanview in the distant past (not sure about its current state of development), but XnView is more powerful and professional, without being more complicated to use. Even as a lightweight image viewer (including raw) it tops any other.

When you need to retouch images, instead of just resampling, resizing or filtering them, I recommend Paint.net. Unless you need a more complicated program like GIMP or Photoshop.

PS I'll split these interesting posts into a new thread in the Technical board :-)