When working with large data (like images) it is wise to keep everything in series (every time a node has more than one wire attached to its output, the msg is cloned. This can be REALLY expensive when working with images).
Next, you dont NEED to join - image tools can output to any msg property you desire. In the example below, i outputed image 1 to msg.image1 and then the 2nd to msg.image2 - this means they are both in the message at the same time and there is no need to fork and join.
Flow...
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