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<counter> elements are covered in the Inquisit 4 language reference in the Inquisit 4 documentation; a better place to look is the Inquisit 3 documentation. I would in addition recommend (re-)writing / modifying your script using Inquisit 3 if that's the version you are ultimately going to use / collect data with. In addition, hundreds of examples using <counter> elements are available in the Inquisit 3 section of this forum: https://www.millisecond.com/forums/Forum12.aspx

The equivalent to

<list slidevalues>
/ items = (5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125)
/selectionmode = random
</list>

<list slideprob>
/ items = (13, 13, 13, 13, 13, 25, 25, 25, 25, 25, 38, 38, 38, 38, 38, 50, 50, 50, 50, 50, 75, 75, 75, 75, 75)
/selectionmode = list.slidevalues.currentindex
</list>

would be

<counter slidevalues>
/ items = (5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125)
/ select = noreplace
</counter>

<counter slideprob>
/ items = (13, 13, 13, 13, 13, 25, 25, 25, 25, 25, 38, 38, 38, 38, 38, 50, 50, 50, 50, 50, 75, 75, 75, 75, 75)
/ select = current(slidevalues)
</counter>

with

<trial sometrial>
/ ontrialbegin = [values.s_value = counter.slidevalues.selectedvalue;
    values.s_prob = counter.slideprob.selectedvalue; ]
...
</trial>

<values>
/ s_value = 0
/ s_prob = 0
</values>

to sample items from the two counters.

I've laid out the most important differences between Inquisit 4 and 3 syntax, including what you need to change to go from 4 to 3, throughout various threads. See e.g. https://www.millisecond.com/forums/Topic16606.aspx and https://www.millisecond.com/forums/Topic15587.aspx#bm15588 .

Oh. Ok. I use the list function quite a bit. I use it to pair up two different numbers with each picture (A winning amount and a winning percentage)

<list slidevalues>
/ items = (5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125,5,8,20,50,125)
/selectionmode = random
</list>

<list slideprob>
/ items = (13, 13, 13, 13, 13, 25, 25, 25, 25, 25, 38, 38, 38, 38, 38, 50, 50, 50, 50, 50, 75, 75, 75, 75, 75)
/selectionmode = list.slidevalues.currentindex
</list>

I could not find anything on counters in the language reference, and simply replacing all the "list" language with counters did not work. Is there a certain way to pair these numbers with these pictures using the counter function?

There are no <list> elements in Inquisit 3 syntax. <list> elements were introduced in Inquisit 4. You need to replace them with the Inquisit 3 predecessor: <counter> elements.