Learning progressions (LPs) are novel models for the development of assessments in science education, that often use a scale to categorize students' levels of reasoning. Pictorial representations are important in chemistry teaching and learning, and also in LPs, but the differences between pictorial and verbal items in chemistry LPs is unclear. In this study, we examined an Ordered Multiple Choice (OMC) LP assessment of explanations of physical properties and processes in matter, that included equivalent verbal and pictorial items. A cohort of 235 grade 7 students that learned the particle model of matter, responded to these assessments and the data was analyzed in terms of their apparent levels of reasoning. We employed two analyses to examine the role of pictorial items in the level-based model of the LP: a polytomous RASCH analysis of the multiple-choice responses, and a verbal analysis of the students' explanations of their choices. We found that our data does not fit a fine-grained, four-level model, but that it does fit a coarse-grained three-level model. In addition, when fitting the data to the three-level model, the pictorial items placed more students in the midlevel than their verbal counterparts. The verbal analysis showed that explanations of selections of pictures that represent a partial, midlevel understanding, were significantly less aligned with the details in the picture, than explanations of the correct, upper-level selections. Finally, the proportions of student explanations of both upper-level choices and midlevel choices that were aligned with the information in the pictures were correlated with the overall difficulty of the items. This suggests that complex pictorial representations of processes are less likely to reveal coherent reasoning.