Notepad++ seems to not have implemented variable-length look-behinds (this happens with some tools). A workaround is to use more than one fixed-length look-behind:
(?<!flecha)(?<!flecha1)\.jpg
As you can check, the matches are the same. But this works with npp.
Notice I escaped the .
, since you are trying to match extensions, what you want is the literal .
. The way you had, it was a wildcard - could be any character.
About the extra question, unfortunately, as we can't have variable-length look-behinds, it is not possible to have optional suffixes (numbers) without having multiple look-behinds.