If nobody has an idea: What I need is to have the possibility to have pictures aligned left from the text in the blockquote. Normally my pictures are aligned middle without text around them.

Why can't you just align the image left, instead of trying to create a new blockquote?

Alternatively, create a css class or id in your stylesheet specifically for the purposes; then edit the html when creating your post and add the class/id to the blockquote.