Hi

. . . . I change not only the selected items (on one row) but also other items (on other rows). . . . .

That is the correct action - by positioning images under others MEP assumes that the relative start of each is set ie the lower images are 'connected' relative to the ones above that they overlap or are inline with as shown below

. . . . I even tried locking the other rows against editing (the "lock" button), but the after changing photo length, the photos remained all in theis previous positions . . . .

Yes they will - the lower track images are 'locked' in position and therefore the 'connected' images cannot move cannot move relative to the images above.

You should end up with gaps between the 'connected' upper images if you are shortening them as shown below

and crossfades between the 'connected' upper images or where they cannot move when you increase the image length as shown below

The first image on track 1 has nowhere to go when increasing the image length so crossfades into the next image.

The end two images on track 1 can move as they please because they have no restrictions on their movement.

This 'connecting' can be seen if you look carefully at the lower track images in the last two examples - they start at the same point relative to their 'connected' image above as shown in the top image.

AFAIK there is no way to change this behaviour - you have to modify your work flow to take this behaviour into account.

HTH

John EB