Changing the duration of a still

I have a still image that will not allow a longer duration to be set after adding it to the timeline. Three methods attempted:
1. Click and drag-The gradient edge is red instead of green
2. Right click-change duration does nothing
3. Right click on another still that is the duration I need, then selecting the option (conform lock with media pool clip) gives this warning: Error: Unable to replace because the replacement clip does not have sufficient content.
QUESTIONS:
1. All stills are the same size and tif format. Why would one still have a designated duration that can not be changed?
2. Why not just use the words: replace clip from media pool, instead of: "Conform lock with media pool clip" so we don't need to search the web to find out how to do such a simple thing. Curious, not whining.
THANKS
Davinci Rsolve Studio 15.0.OB.030
W10 64bit
Intel Core i7-5820K 3.30GHz
6 core
12 threads
32 GD DDR4 RAM
Nividia Quadro K4000 3GB GDDR5 192-bit
SSD-OS
SSD-video editing
SSD-scratch
SSD & HDD-storage
(8 total Hard Drives)
1. Click and drag-The gradient edge is red instead of green
2. Right click-change duration does nothing
3. Right click on another still that is the duration I need, then selecting the option (conform lock with media pool clip) gives this warning: Error: Unable to replace because the replacement clip does not have sufficient content.
QUESTIONS:
1. All stills are the same size and tif format. Why would one still have a designated duration that can not be changed?
2. Why not just use the words: replace clip from media pool, instead of: "Conform lock with media pool clip" so we don't need to search the web to find out how to do such a simple thing. Curious, not whining.
THANKS
Davinci Rsolve Studio 15.0.OB.030
W10 64bit
Intel Core i7-5820K 3.30GHz
6 core
12 threads
32 GD DDR4 RAM
Nividia Quadro K4000 3GB GDDR5 192-bit
SSD-OS
SSD-video editing
SSD-scratch
SSD & HDD-storage
(8 total Hard Drives)