There are quite a few things you need to sort out before BoneJ can give you a sensible estimate of Imax and Imin.
First you have to ensure that the pixels you want to be included in the calculation are foreground (255), and those you want to be ignored are background (0), or alternatively lie within the min and max ranges in the setup dialog. You can also set a rectangular ROI to exclude foreground pixels outside the ROI from the calculation. Make sure you exclude pixels you don’t want in the measurement by setting them to a background value (0 or less than your greyscale min) prior to running Slice Geometry.
The lines that are drawn on the image are not Imax and Imin, they are the maximal and minimal principal axes, around which Imax and Imin are calculated. You will see the principal axes better if you run Slice Geometry on a greyscale image rather than a binarised image.
There is some inconsistency about Imax and Imin and how they relate to maximal and minimal principal axes. You can calculate second moments of area around any arbitrary axes, such as image x and y or anatomical axes. The numerical result Iwhatever axis is the second moment around the axis. This leads to the confusing result that Imax is smaller than Imin, because the same area is on average closer to the longer (maximal) axis than it is to the shorter (minimal) axis.
If your image is calibrated in µm, you should expect Imax and Imin to be very much greater than 1, more like 1×10⁶ or 1×10⁹ µm⁴. You should state the units which are strictly m⁴ but for small samples like this mm⁴ or µm⁴.