Might be better to do blueprint sizing/rotation/etc. here instead of in a separate step. Easier to see misalignments during this phase.
Switch to wireframe views.
In each panel, choose View > Background Image. Load the appropriate image (side, front, etc.) for that panel. Then use the Size and Offset values to fit the blueprint image exactly inside the bounding box I created.
I ran into both rotation and sizing issues. Make adjustments to the image in GIMP, save, and then use the “Reload” button in Blender’s View > Background Image dialog to check it. Make liberal use of Undo in GIMP to experiment with changes to fit the image as closely to the box as possible.
To use GIMP to scale in just one dimension, use Layer > Scale Layer and click the chain button to break the link between the X and Y dimensions. If the blueprint starts to hang off the edge of the image, choose Image > Fit Canvas to Layers to fit the image area to the scaled layer area.
Again, take time to do it right as it will save time in the end.