I want my portfolio thumbnail to bring up a new page to show the whole piece. Will a pop up blocker block the _blank? Should I make it _parent?
I have my browser configured to completely ignore target attributes in the HTML and to open all new pages from JavaScript window.open in a new tab so what you specify makes no difference to me. Those fields are just recommendations that your visitors can ignore.
Most browsers do block the target event unless it’s being used within a frameset on specific frames that match the name of what’s being hooked. 
PS: Why not use a lightbox so it’ll show on the actual page? Popup’s are annoying and the target attribute is deprecated.
It depends on the pop-up blocker. Some will only block Javascript pop-ups, some will only block Javascript pop-ups that run without you clicking on a link, some will block all new windows/tabs, some are configurable by the user.
Why are you messing around with the target anyway? How would _parent help? (That implies that you’ve got a frameset running, and that’s not something you want to admit to round here!) What’s wrong with just opening the link in the same context as the current page - ie, just a plain ol’ <a href=“picture1.htm”> ?
If you make it with progressive enhancement and JavaScript it would still function whether or not the popup spawned and probably please your potential audience more anyway than a “unassigned frame” value.
The odds are if they have installed a popup blocker they wouldn’t appreciate a new window opening in most cases so give them the greater option, or don’t do it.