I'll document this properly tomorrow. I'll give a full description of the steps I went through to analyse and solve this problem, and detail the versions. If this is the wrong place to post this, or if this might better be posted elsewhere, let me know.
for now, let me just give this advice as it might save anyone time and frustration.
I'm upgrading from D6 to D7. Rather than using image legacy and field convert, I decided to migrate from image to imagefield on my D6 installation. the migration script seemed to work fine, image nodes were nicely converted.
after upgrading from D6 to D7 and importing the content etc, only part of the imagefields were populated (I mean, only part of the pictures were visible). It turned out that those pictures stemmed from image nodes (image module nodes) which were not language neutral, and that the field content was marked as 'nl' (in my case).
I had to go back to my D6 install, and with the module language assignment, I had to set the image nodes as language neutral.
So, conclusion: Set your image nodes as language neutral before migrating to imagefield, as this may prevent trouble when upgrading to Drupal 7!
(I am checking my field_revision_field_imagefield table at this very moment, all my previous 'image nodes' survived the upgrade, but the above seems to hold for the attached images as well - as a data item, they retain the language of the node they were attached to and thus aren't displayed with that node. As said, I'll document this properly tomorrow, and I'll try to analyse this 'attached images' issue better)

Comments

joris.verschueren’s picture

joris.verschueren commented
Status: Needs work » Active

I took a bit longer to describe it properly.
Meanwhile, I've found out that I am not the only one who's experiencing this, and that there are more issues with languages and the upgrade of fields. Therefore, I posted the description in #1164852: Inconsistencies in field language handling (# 11)

quicksketch’s picture

quicksketch
he/him
commented

Is there some actionable item here? What can ImageField do to accommodate for this? Is there a reason why Field Convert should not be handling this?

joris.verschueren’s picture

joris.verschueren commented

the problem isn't limited to field convert itself, it seems. the problem is with fields having language attributes, which occurs on multilingual sites, and the way Drupal7 handles these fields. that's what I've understood from the complicated thread I referred to in #1. there, a patch is (to be) provided that resets the language of fields to NULL.
Field Convert could strip language attributes from fields.

quicksketch’s picture

quicksketch
he/him
commented
Status: Active » Closed (duplicate)

So again, nothing to be done here by ImageField module. I'm marking this duplicate of #1164852: Inconsistencies in field language handling as it doesn't seem to have any reason being in the ImageField queue.

joris.verschueren’s picture

joris.verschueren commented

You're very right and I should have done so myself.
I posted this issue as a warning in the first place. With image.module being (almost) discontinued in D7, and Imagefield being integrated in core, I suppose I won't be the only one trying this route. as soon as the language issue is settled, Image -> Imagefield (D6) -> Upgrade to D7 might well become the preferred migration route.
thanks for the work on imagefield!

Joris