Update foreach in foreach c#

This overwrites a.Image for every item in listDetail, therefore you get the last Image:

foreach (var item in listDetail)
{
    a.Image = item.Image;
}

As you have an Id property, I suggest using it for synchronization:

foreach (var item in listDetail)
{
    if (item.Id == a.Id)
    {
        a.Image = item.Image;
        break;
    }
}

I added a break; so you break out of the loop as soon as you found a match.

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