In the Control Panel, what is your setting for inline attachments? Images greater than the inline attachment value will not display inline.

CP -> Display Settings -> General -> Topic & Post Page Settings
Quote
Max File Size for Displaying Uploaded Images Inline
If an image is attached to the post, and if it is smaller than the specified size, it will be displayed in the post. Do not use commas. You may use decimal places, such as 0.5, to specify half of a MegaByte.
Note: 1 MB = 1,048,576 Bytes.

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