In addition of the advice you were given previously, and if you haven't figured it out already, you may want to try this.
If the main image is locked, you may want to unlock it, especially if the main image is in indexed mode, it just wont let you.
You can unlock a locked image by double clicking and clicking OK on the popup you get or if that image is in indexed mode change mode to e.g. RGB to unlock it, then you can drop anything onto it.
The gif above is of main image dropped in indexed mode, and twice tried to drop the speech bubble, then after it fails, just switched mode and voila, its easier for non indexed mode pictures for sure.
And smile, Photoshop has a way of making you go bananas, but, everything has its way.
Hope this helps.