This is a possible bug.
For the Poster Screen - you have these options

screen.SetListStyle("arced-portrait")
screen.SetListStyle("arced-landscape")
screen.SetListStyle("arced-square")
screen.SetListStyle("flat-category")
screen.SetListStyle("flat-episodic")
screen.SetListStyle("rounded-rect-16x9-generic")
screen.SetListStyle("flat-episodic-16x9")

The component reference guide says:
In order to see poster art in the side posters instead of episode numbers, please ensure that the SDPosterUrl and HDPosterUrl are defined for the content and that episodeNumber is not defined for that content. EpisodeNumber overrides the poster URL.

I am not specifying an episode number, because I want to have it display my very own individual episode graphics.
All of them except type "flat-episodic" load the hdposterurl or sdposterurl for the items to the left and right of the current selection. "flat-episodic" only displays the image for the currently selected item and the left and right sides are blank squares. What makes that style special to do that behavior? Strangely enough, if I delete/remove the "hdimg" and "sdimg" values from the xml, it will display the HDPosterURL or SDPosterURL for the currently selected item - weird huh?

I use these routines:
Function init_show_feed_item() As Object
o=CreateObject("roAssociativeArray")
o.ContentId=""
o.Title=""
o.HDPosterURL=""
o.SDPosterURL=""
o.ContentType="episode"
o.ContentQuality="SD"
o.StreamQualities=CreateObject("roArray",5,TRUE)
o.StreamBitrates=CreateObject("roArray",5,TRUE)
o.StreamUrls=CreateObject("roArray",5,TRUE)
Return o
End Function

Function parse_show_feed(xml As Object,feed As Object) As Void
showCount=0
showList=xml.GetChildElements()
For Each zzz In showList
o=CreateObject("roAssociativeArray")
o.ContentId=""
o.Title=validstr(zzz.title.GetText())
o.ShortDescriptionLine1=o.Title
o.sdImg=validstr(zzz@sdImg)
o.hdImg=validstr(zzz@hdImg)
o.HDPosterURL=validstr(zzz@hdPosterUrl)
o.SDPosterURL=validstr(zzz@sdPosterUrl)
o.ContentType="episode"
o.ContentQuality="SD"
o.StreamQualities=CreateObject("roArray",5,TRUE)
o.StreamBitrates=CreateObject("roArray",5,TRUE)
o.StreamUrls=CreateObject("roArray",5,TRUE)
o.StreamFormat="mp4"
o.HDBranded=false
o.IsHD=false
o.ContentType="episode"
For idx=0 To 4
e=zzz.media[idx]
If e<>invalid
o.StreamBitrates.Push(strtoi("1000"))
o.StreamQualities.Push("SD")
o.StreamUrls.Push(validstr(e.streamUrl.GetText()))
End If
Next
showCount=showCount+1
feed.Push(o)
Next
End Function

This code results in the following structure for all items - with data unique to each item

shortdescriptionline1: Test App Name
contentid:
sdposterurl: http://mysite.com/image.png
ishd: false
hdimg: http://mysite.com/image.png
streamformat: mp4
streamurls: <ROT:roArray>
contentquality: SD
streamqualities: <ROT:roArray>
streambitrates: <ROT:roArray>
sdimg: http://mysite.com/image.png
hdposterurl: http://mysite.com/image.png
title: Test App Name
contenttype: episode
hdbranded: false

Every other style type works as I like - just flat-episodic has slightly different item layout that I want to use without building a custom poster screen by hand. Again, "flat-episodic-16x9", "rounded-rect-16x9-generic", etc etc all display perfectly fine with the custom episode number images to the left and right of the current selection by simply changing the list style, but "flat-episodic" turns them into empty gray squares with no graphics -- if I set an episode number for the items, then it displays a generic number font like it's supposed to, but that's what I'm trying to replace with a custom image for the number.

For an example of what I'm talking about - look at the Ace Broadcasting channel - the flat-category screen shows graphics for each category for each show all at the same time. Pick one and go to the episode list which is using flat-episode and only the current selected episode shows up with gray squares on whatever isn't currently selected. (Just figured I could probably make this a bit more colorful)