Regardless of there being a minigame or not, when loading significant amounts of information into memory at once, the screen can freeze for a period of time with no feedback at all for the user, not even a loading animation. This is bad design, as it can lead the user to assume the program has crashed.
Even if for a short period of a few seconds, it can be disconcerting. For longer periods, the user may even force quit.
Anyways, IMO minigames during loading screens are the greatest thing ever. Especially for games that have relatively more instances cases of loading during any given session. For me the first time I encountered it was on the PS2 version of Okami, and it really made a positive impression on me.
skymen - It could be wasteful of memory to keep a heavy image like that on every layout and not use it. But that really depends on your project and if you are pushing your limits. Using the techniques described, it would be better to change to an intermediate loading layout (thus unloading the previous one from memory), then load the heavy image by request, then switch to the new layout. The intermediate layout could have your loading animation or minigame.