There isn't a way to make the tracing image fully opaque, but you could do this instead:
After loading your tracing image, open the tracing image panel, tap the menu button ( bottom right ) and choose 'convert to paint' to convert the tracing image to paint on a single layer.
Then apply your media to a layer above. Depending on what you want to acheive, you could use the 'tint' or 'multiply' blend mode for the upper layer. You can change the blend mode in the layers panel by selecting the layer you want to change it for then tapping 'blend mode: normal' and selecting from the list.
Alternatively, you could start your painting from the gallery by choosing to import the photograph directly as a new painting, then add a layer similar to what I described above.
ArtRage for iPad supports png and jpg export, but doesn't have a tiff option.
I hope this helps!
Resident Bug-Hunter and Support Guy