Place clip in timeline.
Click MODIFY (while the clip/image is high-lited)
Scroll down to the bottom of that first open tab.
Near the bottom you will see OPACITY.
That slider controls the transparency.
Moving the slider while anywhere on the clip will show the result, but you'll need to click on that spot, within the MODIFY timeline, and on the topmost OPACITY, to make a key-point, and KEEP your change. You can alter the transparency by adding keypoints as many times as you need. While you're there, you'll also see the FADE OUT/IN option. I use that very often, rather than the fade transition. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

Thank you very much!
the clips in the particle room, with several layers, do not have that option though? but it's good enough for me.

The particle room is different.
If you need to alter a particle clip...pre-produce!
Create your particle, produce a video file from it, and then bring that produced file into your project. You can then adjust transparency/opacity.
You MIGHT need to lay a green-screen (green image) behind your pre-production (a track ABOVE your particle) and CHROMA-KEY that out later, so the background will be transparent, rather than black. If you try to CHROMA-KEY out the black it will also remove any black portions throughout the clip, poking holes in weird places. HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
Nvidia GTX 960(4GB)/16GB DDR3/
Canon Vixia HV30/HF-M40/HF-M41/HF-G20/Olympus E-PL5.
Tape capture using 6 VCR, TBC-1000, Elite BVP4+, Sony D8 camcorder with TBC.
https://www.facebook.com/BarryAFTT

thanks!