Thanks for posting the image.
For calcium imaging, you want average intensity within the cell, so you should check "Mean Gray Value." For your graph, you probably want to calculate F/F0 (mean fluorescence at time t divided by the mean at time 0) for each cell.
Notice that your rectangular ROIs include area both inside and outside the cell. This will reduce your apparent intensity and % change, and introduce variability due to varying amounts of background in each ROI.
For the most accurate & reproducible results,
1. Use the freehand ROI (looks like a bean) to draw exactly around the cell.
2. Measure the mean intensity of a background area and subtract this value from all mean intensity values before graphing the data.
Also, make sure the cells don't move outside the ROI during the time series.