Jose
I have added spaces in some of the text here so you will be able to read this, but using your start pic example, to show full size remove this first bit from your text:
[ u r l=https://gallery.kitmaker.net/showphoto.php/photo/474609]
so your start is from the [ I m g ] :
[ I m g ]https://gallery.kitmaker.net/data/500/thumbs/IMG_78282.JPG[/ I m g] [/ ur l]
...then remove the [/ u rl] at the end, after the [ / I m g ]
Then with the section you are left with, remove the part
/thumbs
so you will then have the link text to the full size pic being:
[ I m g ]https://gallery.kitmaker.net/data/500/IMG_78282.JPG[/ I m g]
and without the blanks in the img tags I put in, your full size pic will show like this:
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