QB Issue Resolution:

Invoice attachment to Invoice is located at bottom left.

See screenshot photo for reference.

You can even upload batch of images from (Gear (upper-corner) > Attachments under Tools). You can also Create Invoice from here as well.

original (16)

Resolution for Issue 'How do I add an attachment to my customer invoice? There used to be a section in QuickBooks Online for this on each invoice.' available: Yes (Solved).
Source: Intuit Community forum.

How do I add an attachment to my customer invoice? There used to be a section in QuickBooks Online for this on each invoice.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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